The invention relates to a high-pressure fuel infection pump and to a. fuel Injection system comprising a high-pressure fuel injection pump.
Such high-pressure fuel injection pumps and fuel injection systems comprising such pumps are normally used for pressurizing fuel and for delivering it for injection into an internal combustion engine.
Rising prices of crude oil-derived fuels and fears of its Imminent shortages have in recent years led to further developments in production processes of alternative fuels and internal combustion engines for their use. One of the potentially important alternative fuels that can be effectively produced from a variety of stocks including biomass is dimethyl ether (DME). DME, with its soot-free combustion and high cetane number, is very well suited for diesel-type internal combustion processes. However, DME has a relatively high volatility (compared with normal diesel fuel) and, therefore, has to be pressurized, to approximately 5 bar in order to be liquid at room temperature. There are a number of advantages of having fuel supplied in liquid form for injection Into a diesel-type internal combustion engine, and thus fuel injection equipment (FIE) suited for DME or other similarly volatile fuel should be specially designed to prevent unwanted vaporization of the fuel or to effectively remove vapour cavities from such parts of the injection system where vapour can disrupt normal operation.
High-volatility fuels can be prevented from boiling by selecting a higher pressure and/or lower operating temperature. hi a particular engine application, fuel must be supplied to the injection pressure-generating part of the FIE at an optimum combination of fuel feed pressure and temperature. In selecting that optimum combination, the cost and complexity of the fuel feed system are significant controlling factors. Imbalance towards an higher feed pressure would, whilst allowing operation, at elevated temperatures, increase system cost, complexity, weight and energy consumption, at the same time itself causing extra fuel heat-up. On the other side, lowering fuel feed temperature beyond a certain limit would not be feasible, for obvious reasons.
In order to shift the combination between the fuel feed pressure and temperature towards more energy-efficient and less expensive balance, a number of design solutions are used in prior art systems. One such solution is recirculation of fuel feed flow throughout the system at a rate exceeding the rate of fuel consumption by the engine. By this means, equalization of temperatures between the coldest (such as fuel tank) and the hottest parts of the system is achieved and therefore the risk of local vaporization is reduced. Another approach is thermal insulation of parts of the system that are most exposed to heat sources.
For a number of reasons, the most suitable type of FIE for high-volatility fuels such as DME, is a common rail system well-known in the art. Such a system usually incorporates a multi-plunger high-pressure pump that supplies fuel under high pressure to injectors via a common rail. Over many years of development and use in the diesel engine industry, the typical high-pressure pump of a common rail system has evolved to incorporate the output control concept based on the inlet metering principle and fixed plunger displacement. This makes use of a single electronically controlled inlet metering valve (IMV) to define the amount, of fuel reaching the plungers during their respective inlet strokes. At a given feed pressure, varying the valve's restriction varies the output of the plungers which, in. turn, at a given engine consumption, varies the common rail pressure.
The single IMV, fixed, displacement route-plunger type of pump is widely accepted in common rail FIE on the grounds of its relative simplicity as compared to variable-displacement pumps. However, when high-volatility fuels are used, improvements to such a pump are necessary to ensure reliable performance across the full operating range of an engine. This is because in this type of pump there is a significantly large volume of fuel, situated in the suction channel leading fuel from the IMV up to the plungers, the pressure in which most, of the lime needs to be controlled to a lower level than the available feed pressure for the purpose of pump output control. By definition, this volume of the suction channel is then separate from the recirculated fuel feed circuit, and thus vulnerable to overheating and fuel vaporization. One way of alleviating this problem is to thermally insulate the suction channel from the surrounding parts. This works well down to a certain minimum fuel flow in the suction channel, but still the vaporization can take place when the flow is under that minimum, or when there isn't flow at all, for a relatively long period of time. The operating conditions requiring extremely low or no output from the high-pressure pump and correspondingly low through-flow in the suction channel, can occur frequently and it can be necessary for the pump to change from that to full output almost immediately, such as when a hot engine needs to start or when high torque is demanded after a period of engine braking. Once fuel is allowed to vaporize in the suction channel, the hydraulic efficiency of the pump is dramatically reduced and it can then take a long time before the pump can provide full output, unless special, action, is taken.
It is known in the art to provide a separate bleed function for the suction channel in order to be able to remove vapour from that channel instead of trying to liquefy it. The latter is usually a much longer process than vaporization, and to provide for liquefaction in a relatively hot pump, the feed pressure system would need to be capable of relatively high pressure and would therefore be unnecessarily complex, heavy and expensive. Bleeding the suction channel which is thermally insulated from the hot pump body, with a relatively cold and low-pressure fuel from the feed recirculation line, can simultaneously and effectively cool that channel and restore normal operation of the pump much easier. In that and other prior art systems, the bleed valve for the suction channel is electronically controlled, either by a separate electrical actuator or by means of an extended range actuator that simultaneously controls the bleed valve and the IMV. This adds cost because of the actuator(s) and. the need of a more sophisticated control algorithms for the engine management system (EMS).
In prior art systems, for the bleeding of the suction channel to be effective, a separate return line is provided between the bleed valve and the downstream of the feed pressure-setting restriction in the re-circulation circuit. This is necessary because otherwise the bleed path becomes parallel to the main re-circulation path that is designed to have as low restriction as possible up to the teed pressure-setting restriction, and in that case the pressure head across the suction channel and the bleed valve would be too small to drive the vapour out of the pump. In systems where the pressure-setting restriction has to be positioned relatively far away from the pump, the extra cost and complexity caused by the need for such separate return line, can be significant.
It is desirable to provide a fuel injection system comprising a high-pressure fuel injection pump that is less vulnerable to vapour formation in the fuel.
It is also desirable to provide a system for injection of high-volatility fuels, as for instance DME, for internal combustion engines.
It is also desirable to provide a system of injection of high-volatility fuels, such as DME, that has an improved performance and a lower cost, compared to prior art systems.
A general advantage of an aspect of the present invention is that it ensures automatic and effective removal of vapour that may form in the fuel and disrupt normal operation of the fuel injection pump, thereby enhancing the reliability and robustness of the control of delivery of pressurized fuel for injection into the internal combustion engine.
According to an aspect of the invention, a fuel injection system for pressurizing fuel and delivering it for injection into an internal combustion engine is proposed, wherein said system comprises a high-pressure fuel injection pump with an inlet (for receiving fuel from e.g. a fuel tank), at least one plunger (that pressurizes the received fuel and delivers it to injectors for injection into the internal combustion engine), a suction channel positioned between the inlet and the at least one plunger (thereby connecting the inlet of the high-pressure fuel injection pump with the inlet port of the at least one plunger), and a bleed valve connected to the suction channel for removal of vapour that may in certain circumstances form in the suction channel, and for cooling down the suction channel in order to prevent conditions causing further vaporization of the fuel in the suction channel. The fuel injection, system further comprises a return port (for leading any leakage and recirculated fuel back to a fuel tank). To ensure effective removal of vapour by the bleed valve, a selector valve is installed between the inlet and the return port.
In a preferred embodiment of an aspect of the invention, the selector valve is able to open or close the hydraulic connection between the inlet and the return port. An inlet -metering valve (IMV) is installed between the inlet and the suction channel for fine control of output of the high-pressure pump. For conditioning of incoming fuel that the IMV receives, a re-circulation of pressurized fuel is provided by means of a feed pump supplying fuel to the inlet, and an hydraulic flow restriction downstream of the return port. Advantageously, a non-return valve is employed to function as the bleed valve, having its inlet connected to the suction channel. The selector valve is actuated by a return spring and the pressure developed by the plunger, such that when the plunger pressure is below a p re-determined level, the spring closes the selector valve, and when the plunger pressure is above said level, the spring is compressed and the selector valve is open. The selector valve ensures the effective removal of vapour and cooling of the suction channel by selecting the series connection of the inlet, the suction channel and the return port, such that full re-circulation flow is established via the suction channel. Preferably, the said predetermined level of plunger pressure is chosen to be somewhat below the minimum setpoint pressure at which the engine is designed to operate, but at the same time above the pressure that the high-pressure pump is able to maintain when the fuel in the suction channel is vaporized. Then, in the event of uncontrolled vaporization in the suction channel leading to uncontrolled plunger pressure drop, the selector valve will close the connection of the inlet to the return port and establish full re-circulation flow through the IMV, the suction channel and the bleed non-return valve, thereby quickly removing the. vapour already formed and cooling the suction channel to prevent further vaporization. By this means, the pump will regain full output and quickly restore its outlet pressure to a given setpoint, at which instant further rise of output pressure would be cheeked by the IMV controlled by the EMS, Simultaneously, the increased plunger pressure will compress the spring to re-open the selector valve, such that the re-circulation flow will be resumed around the inlet of the IMV to ensure absence of hot spots and reliable delivery of liquid fuel to the inlet.
The invention, according to an aspect thereof advantageously allows a common teed pressure-setting restriction to be positioned downstream of the return port, because the selector valve allows establishing a series connection of the inlet and the suction channel to the return port, such that the bleed function is not reteriorated by by-pass recirculation flow from the inlet to the return port.
In another embodiment of an aspect of the invention, any or both of the bleed and selector valves are made electronically controlled, such that it is possible to control the bleed function with the EMS.